Hmm....is there another band that is more successful? AC/DC is probably the closest but their new album is marginal at best.

You said rock. That's quite a big genre.

EDIT: I'll be the first to admit that some of the logic I originally used in this post is flawed. So let's just think about something here. Rock is a broad genre. How about 2008's highest grossing tours, which were Bon Jovi (210.6 million) and Bruce Springsteen (204.5 million) ? Metallica is definitely successful, but I highly doubt they are the most successful with such a broad genre to compete.